The present invention relates to spread spectrum communications systems and in particular, to the pseudo-noise signal (PN signal hereinafter) synchronizing method, the gain control method and the synchronizing judgement method, and is, for example, suited to application to optical communications, radio communications and power-line carriers.
The known literature, which discloses the conventional technology relating to the present invention, includes "Spread Spectrum Communications Systems" (H. Yokoyama, Scientific Technology Press (KAGAKUGIJUTSU SHUPPAN-SHA); 1988, pp. 300-304).
In a conventional spread spectrum communications system, it is necessary to have phase synchronization on the receiving side between the reverse spread code and the received signals. A synchronization circuit using a non-coherent delay locked loop (DLL) is used to achieve phase synchronization. This synchronizing circuit uses a PN generator inside the DLL to obtain a phase differential signal for sync control from the correlations (first correlation) between the received signal and the PN signal that are several cycles faster than the timing of a clock signal and from the correlation (second correlation) between the receive signal and the PN signal that are several cycles slower than the timing of a clock signal. However, such a DLL has a problem in that there is an adverse influence exerted on the tracking characteristic thereof when the balance between the gains of the two correlators is upset.
FIG. 1 is a view of a configuration of a non-coherent delay locked loop (DLL), and illustrates the case for a 1 .DELTA.-DLL which uses a pN (pseudo noise) signal displaced 1 cycle from the synchronizing loop.
In this figure, the DLL is comprised of the PN signal generator (pseudo-noise generation unit) 61, the VCC (voltage control clock generator) 62, the OSC (local oscillator) 63, the multipliers 64-67, the BPF (band pass filters) 68, 69, the correlators 70, 71, the envelope detectors 72, 73, and the adder 74.
The PN signal generator 61 is driven by the timing of the control clock 62 and generates PN signals. Here, when signals are generated a 1/2 cycle faster PN signal (called "nearly signal" hereinafter) and PN signals 1/2 cycle slower (called "late signal" hereinafter) are taken from the PN signal generator 61. In FIG. 1, "a" shows the early signal and "b" shows the late signals.
The multipliers 64, 65 respectively multiply the early signals "a" and the late signals "b" with the local oscillator signals generated by the local oscillator 63, and place them on the carrier band. The multiplier 67 multiplies the early signal which has been placed on the carrier band and the received signal, and these signals are filtered by the band-pass filter 69 so that only the intermediate frequency components thereof remain. At this time, a simultaneous correlation is achieved between the intermediate-frequency element and the PN signal of the received signal y(t). In the same manner, the late signals are multiplied by the multiplier 66 and filtered by the band-pass filter 68 so that only the intermediate-frequency components remain, and correlation between the intermediate frequency component and the PN signals is achieved. Futhermore, the output of the correlators 70, 71 is taken in the envelope detectors 73 and 72 as the amplitude of the correlators. The adder 74 obtains a difference signal between these two signals. The difference signal is fed back as the control signal for the control clock 62.
In addition, in the prior art Japanese Patent Laid Open Application No. 92-57174 "Synchronizing Method for PN signals in a Base band" the fact that the correlation value between PN signal of the Manchester code and a NRZ (Non Return to Zero) code has an S-shaped characteristic curve is used, and this characteristic is used in controlling the control clock. When this is done, the information signals cause the correlation value to have positive and negative values thereof reversed, and so the correlation value of the NRZ code pairs are multiplied with this correlation value so that the phase difference of the PN signals are always decreased. Moreover, as shown in FIG. 2 (A), the NRZ code is a code which has logical code value 0 corresponding to a negative voltage and logical code value 1 corresponding to a positive voltage. In addition and as shown in FIG. 2 (B), a Manchester code is a code which has logical code value 0 corresponding to the status where there is change from positive to negative, and logical code value 1 corresponding to the status where there is change from negative to positive.
In addition, there has also been proposed a "Spread Spectrum Receiver" in Japanese Patent Laid Open Application No.91-235541. This publication relates to a gain control method in spread spectrum communications.
Spread spectrum communications frequently have a C/N (carrier to noise) ratio of less than one due to that communications are performed in a status where the carrier-frequency level is smaller than the noise level. Therefore,it is not only necessary to apply AGC (Auto Gain Control) for the level of the received signals, as in the case of normal radio communications, but also it is necessary to apply AGC for the level of signals which are the reverse spread of the received signals.
In Japanese Laid Open Patent Application No. 92-35239 there is disclosed a "Synchronizing Establishment Judgement Circuit". The circuit of this publication relates to a synchronizing judgement method in pseudo-noise signal synchronization.
The synchronization circuit for the PN signals used in spread spectrum communications exhibits only a tracking characteristic with respect to phase errors of .+-.1/2 cycle or .+-. some several cycle portions of PN signals, and it is not possible to establish synchronization with respect to phase errors larger than this. Accordingly, when initial synchronizing is performed or when there is a synchronization step-out, it is necessary to detect that there is a step out of synchronization, and to slide the PN signal until tracking by the synchronizing circuit becomes possible. A synchronous judgement circuit is used to detect that there is a synchronization step-out.
Spread spectrum communications using the conventional direct sequence (DS) method obtain synchronization of the PN signal mainly by using a DLL such as shown in FIG. 1. However, a conventional DLL obtains the difference of the two correlators 70, 71, so there is a disadvantage in that the synchronizing tracking characteristic deteriorates if the balance of the gains of the correlators 70, 71 is upset.
In addition, a DLL performs synchronizing in the state where the phase of the PN signal has slipped by only 1/2 cycle, and so reverse spread is achieved by separately taking from the synchronizing loop those PN signals of the same phase as the received signals and then reverse spreading the received signals again. Then, the DLL performs AGC control by detection of the level of these reverse spread signals, and subsequently performs synchronizing judgment.
Furthermore, a method for performing AGC and synchronizing judgment by referring to the level of a conventional reverse spread signal has an extremely sharp correlation characteristic curve for the PN signal, as can be seen in FIG. 3 (A), and so, jitter of the phase of the PN signals causes the level of the reverse spread signal to fluctuate largely, thus resulting in a disadvantage in that deterioration of the AGC and synchronizing judgement characteristics occurs.
Furthermore, in "A Modified PN Code Tracking Loop" (R. A. Yost, R. W. Boyd, IEEE Transactions on Aerospace and Electronic Systems. 1980), there is disclosed a PN signal synchronizing loop which multiplies together PN signals of the same phase (called "on-time" signals hereinafter), a correlation value for the received signals, and one other correlation value.
However, "A Modified PN Code Tracking Loop" does not disclose a "one other correlation value" which results from multiplying the received signal and the difference signal between the early signal and the late signal, and the correlation signal between the received signal, the PN signal and clock signal.
Furthermore, the optimum bandwidth of a band spectrum of a correlator such as a DLL synchronizing loop, a Tau dither loop and a MCL (Modified Code Loop) for PN signals is disclosed in "Non-coherent pseudo-noise code tracking loop of Spread Spectrum Receiver"(M. K. Simon, IEEE Trans. Communication. vol. COM-25, No.3, pp. 327-345, March, 1977) and "A Modified PN Code Tracking Loop; Its performance analysis and comparative evaluation" (R. A. Yost and R. W. Bond, IEEE Trans. Communication. vol. COM-30, No.5, pp. 1027-1036, May, 1982). However, the values determined in this literature are optimum values which relate only to these three loops and there is no mention of the present invention's optimum width for a band pass filter for a PN synchronizing loop.